Carbon is a useful absorber because it is panchromatic and has high optical density per weight coverage. It has the problem of causing field blooming when used in close proximity to segmented electrodes in a cholesteric liquid crystal display.
Carbon can also interfere with process steps in the manufacture of displays as for example when UV curable inks are used for conductive electrodes and traces and UV curable dielectrics are used to make insulating layers. The UV absorption of carbon reduces the actinic light in the curing unit and the unwanted UV and IR absorption causes excessive heating that can thermally transition the liquid crystal.
Currently, information is displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically written areas to carry ticketing or financial information, however magnetically written data is not visible.
Media systems exist that maintain electronically changeable data without power. Such system can be electrophoretic (Eink), Gyricon or polymer dispersed cholesteric materials. An example of electronically updateable display can be found in U.S. Pat. No. 3,600,060, that shows a device having a coated then dried emulsion of cholesteric liquid crystals in aqueous gelatin to form a field responsive, bistable display. U.S. Pat. No. 3,816,786 discloses a layer of encapsulated cholesteric liquid crystal responsive to an electric field. The electrodes in the patent can be transparent or nontransparent and formed of various metals or graphite. It is disclosed that one electrode must be light absorbing and it is suggested that the light absorbing electrode be prepared from paints containing conductive material such as carbon.
Fabrication of flexible, electronically written display sheets is disclosed in U.S. Pat. No. 4,435,047. A substrate supports a first conductive electrode, one or more layers of encapsulated liquid crystals, and a second electrode of electrically conductive ink. The conductive inks form a background for absorbing light, so that the display areas appear dark in contrast to non-display areas. Electrical potential applied to opposing conductive areas operates on the liquid crystal material to expose display areas. Because the liquid crystal material is nematic liquid crystal, the display ceases to present an image when de-energized.
The patent discloses the use of dyes in either the polymer encapsulant or liquid crystal material to absorb incident light. The dyes are part of a solution, and not solid submicron particles. The patent further discloses the use of a chiral dopant in example 2. The dopant improves the response time of the nematic liquid crystal, but does not create a light reflective state. The display structures disclosed are not bistable in the absence of an electrical field.
U.S. Pat. No. 5,251,048 discloses a light modulating cell having a polymer dispersed chiral nematic liquid crystal. The chiral nematic liquid crystal has the property of being electrically driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal conic state. Chiral nematic liquid crystals, also known as cholesteric liquid crystals, have the capacity of maintaining one of multiple given states in the absence of an electric field. Black paint is applied to the outer surface of rear substrate to provide a light absorbing layer outside of the area defined by the intersection of segment lines and scanning lines.
U.S. Ser. No. 2002/0089625 to Grupp discloses disposing, in reflective liquid crystal display devices, the absorbent black layer which is usually deposited on the lower face of the back substrate, and arranging it at a higher level than the level of said back substrate. In this way, the number of so called parasite reflections or back scatter of the incident light at the interfaces between two materials or mediums of different indices is reduced. This allows the display contrast to be improved. Grupp discloses a polymer dispersed liquid crystal device having a first group of transparent electrodes and a second group of electrodes. A black colored absorbent layer made of electrically nonconductive material that is an insulating lacquer, Heatsinkpaste® HSP 2741 by Lack Verke GmbH. The invention of Grupp requires separate processes for depositing the absorbent black layer and the second conductor. In addition, there is no mention of providing an index match between the absorbent black layer and the polymer used in the polymer dispersed liquid crystal layer. It would be advantageous for the absorbent layer to be disposed between the polymer dispersed liquid crystal material and the second electrode, in order to maximize the light absorption.
U.S. Pat. Application Serial No. 2004/0032545 discloses a thin, dark light absorbing layer between two thinly spaced, parallel electrodes operating on polymer dispersed cholesteric liquid crystal displays, in which, if the light absorbing layer for a display having polymer dispersed cholesteric liquid crystals is not field carrying and not electrically conductive, it is possible to position such layer between electrodes to provide improved image quality. Accordingly, the light absorbing layer does not carry a field beyond limits defined by the intersection or overlap of the two electrodes. The disclosed display has polymer dispersed liquid crystals, comprising a transparent substrate, a polymer dispersed cholesteric liquid crystal disposed over the substrate and defining first and second surfaces, a first transparent conductor disposed over the first surface of the state changing layer, a second conductor on the second surface of the state changing layer, and a nonconductive, non-field spreading layer comprising a submicron pigment and binder disposed between the polymer dispersed cholesteric liquid crystal layer and the second conductor to provide a light absorbing layer. Fine, preferably submicron, particles of pigment in a binder provide an electro-chemically stable light absorber that maximizes light absorption in the pigment-containing layer, while preventing field spreading beyond the perimeter of the second electrode.